Fibrous reinforced hoses used for the conveyance of fluids under pressure are well known. Generally, such tigue under pressure pulsation and flexing conditions over equivalent hose constructions that utilize metallic wires for the reinforcing material.
Until recent time, fibrous reinforced hose has been limited in its pressure carrying capacity because of the strength limitations inherently associated with fibers made from conventional materials such as rayon, aliphatic nylon, cotton and poly(alkyline-terephthalate)ester, such as poly(ethylene-terephthalate)ester sold by E. I. du Pont de Nemours and Co., under the trademark "Dacron", and the like. Adding layer upon layer of such conventional fibers about the core tube of a hose is not the solution for significantly increasing its pressure carrying capacity for it has been found that such hoses tend to become overly large in their outer diameters and that the efficiency of each additional layer of reinforcement decreases as its radial distance from the outer surface of the core tube of the hose increases.
The development of fibers made from aromatic polyamide materials by personnel of E. I. du Pont de Nemours during the late 1960's and early 1970's such as disclosed in the U.S. Pat. No. 3,600,350, has for the first time provided the hose making industry with a fiber having significantly improved strength characteristics. The aromatic polyamide fiber is marketed by E. I. du Pont de Nemours and Co. under the trademark "Kevlar" and is known in the trade variously as "Fiber B" and as an "aramide" filament. Fibers spun from "Kevlar" aromatic polyamide have a high tensile modulus and are composed of monofilaments prepared from the reaction mixture of an aromatic dicarboxylic acid and an aromatic diamine such as terephthalic acid or anhydride and p-phenylene diamine. Alternatively, the aromatic polyamide may be an aromatic aminocarboxylic acid, such as 4-aminobenzoic acid. Fibers made from "Kevlar" generally exhibit a tenacity of from about 12 to about 25 grams per denier, with an average of about 20 grams per denier, and an elongation at break of about 2% to about 8% with an average of between 2% and 4%.
In contrast, fibers made from "Dacron" or aliphatic nylon filaments generally have a tenacity in the range of 8 to 12 grams/denier and an elongation at break in the range of about 8% to about 12%. Thus fibers made from "Kevlar" are generally twice as strong as the aforementioned conventional fibers and exhibit about one-half of the elongation at break of such fibers. The term "aromatic polyamide" fibers as used herein means a synthetic fiber made from aromatic polyamide having a tenacity in substantially untwisted form of from about 12 grams/denier to about 25 grams/denier and an elongation at break of about 2% to about 8%.
It is also known to those familiar in the synthetic fiber art that the amount of twist in the fiber, expressed in turns/inch, has an effect upon the tenacity of the fiber. Generally, the tenacity of a fiber will increase up to a point as the amount of twist is increased and once that point is exceeded will then begin to decrease as the amount of twist becomes increasingly excessive. E. I. du Pont de Nemours discloses such a phenomena related to "Kevlar" on Page 7 of their Bulletin K-3 dated December 1978.
Also to be considered in the art of constructing high pressure hoses is whether to utilize one or more helically wound or braided reinforcement layers or combinations thereof. Generally, one or more layers of helically wound reinforcement has been used in the past to provide hose capable of withstanding extremely high pressure. Helically wound layers of reinforcement are believed to provide greater strength over an equivalent amount of braided reinforcement due to the fact that adjacent fibers in each layer are able to lay more intimately against each other in contrast to the "in and out" pattern associated with a braided construction. An example of an early use of combined spiral wrapped and briaded reinforcement in high pressure hose can be found in U.S. Pat. No. 1,973,756 in which is disclosed a flexible hose capable of withstanding extreme pressures having a braided reinforcement sheath about the core tube and a helically wound reinforcement sheath about the braided sheath with a yieldable material disposed between the reinforcement layers. The disclosure however is directed primarily towards the use of metal wire as the reinforcement material and does not take into account the effect that twist may have on fibrous reinforcements.
Although it has been found that aromatic polyamide fibers can be used to provide hoses designed to withstand extremely high pressure, it has also been found that such hoses, for some not yet understood reason, exhibit poor resistance to flexing and pressure pulsation fatigue and that something more is required than merely increasing the twist of such fibers to provide an optimum level of tenacity.
It has been discovered that a significant improvement in the resistance to flexing and pressure pulsation fatigue of high pressure hoses utilizing aromatic polyamide can be achieved where there is a prescribed relationship between the amount of twist associated with successive layers of such fiber about the core tube of the hose. Thus, the present invention contemplates a means by which the aforementioned strength of aromatic polyamide fibers, such as "Kevlar", can be used to advantage and further provides a means of significantly improving the working life associated with the use of such hose.